Comment on "High-pressure spark plasma sintering (SPS) of transparent polycrystalline magnesium aluminate spinel (PMAS)" by M. Sokol, S. Kalabukhov, M. P. Dariel, N. Frage [J. Eur. Ceram. Soc. 34 (2014) 4305]

The validity of the Hall-Petch relationship for transparent spinel ceramics with grain sizes of 0.2-0.1 mm is confirmed here in context with data for coarser microstructures >0.3 mm and up to 200 mm whereas there is no further hardness increase on the transition to nanoscale spinel microstructures. Although such cubic ceramics do not suffer birefringent scattering, all active optical losses are subject to a strong influence of the thickness. Most technical applications of transparent spinel request components thicker than 1-2 mm and need, therefore, very high in-line transmission data when measured on thinner samples.